nitrogen6x: Fix RAM size variable

Fix the following build error when buildig nitrogen6s1g:

nitrogen6x.c:89:17: error: 'CONFIG_DDR_MB' undeclared (first use in
this function)
nitrogen6x.c:89:17: note: each undeclared identifier is reported only
once for each function it appears in
Reported-by: NAlbert Aribaud <albert.u.boot@aribaud.net>
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>

boards.cfg

@@ -260,7 +260,7 @@ nitrogen6dl2g                arm         armv7       nitrogen6x          boundar
 nitrogen6q                   arm         armv7       nitrogen6x          boundary       mx6		nitrogen6x:IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6q.cfg,MX6Q,DDR_MB=1024
 nitrogen6q2g                 arm         armv7       nitrogen6x          boundary       mx6		nitrogen6x:IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6q2g.cfg,MX6Q,DDR_MB=2048
 nitrogen6s                   arm         armv7       nitrogen6x          boundary       mx6		nitrogen6x:IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6s.cfg,MX6S,DDR_MB=512
-nitrogen6s1g                 arm         armv7       nitrogen6x          boundary       mx6		nitrogen6x:IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6s1g.cfg,MX6S,SDRAM_MB=1024
+nitrogen6s1g                 arm         armv7       nitrogen6x          boundary       mx6		nitrogen6x:IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6s1g.cfg,MX6S,DDR_MB=1024
 cm_t35                       arm         armv7       cm_t35              -              omap3
 omap3_overo                  arm         armv7       overo               -              omap3
 omap3_pandora                arm         armv7       pandora             -              omap3